1. Field of the Invention
The present invention relates generally to an optical storage apparatus for optically recording and reproducing information on/from a removable medium such as an MO cartridge, and more particularly to an optical storage apparatus capable of appropriately dealing with any zero-cross error detection arising from ID fields or noises of the medium in a seek control in which the number of tracks are counted in response to the zero-cross detection of a tracking error signal.
2. Description of the Related Art
A lot of attention is being paid to optical disks as storage media, which are indispensable elements in recently rapidly developing multimedia. In case of e.g., a 3.5-inch MO cartridge, there are provided in recent years high density recording media having a memory capacity of 540 MB or 640 MB, in addition to a conventional 128 MB or 230 MB types.
An optical storage apparatus known as an optical disk drive for making use of such an MO cartridge medium performs, upon a reception of a write instruction or a read instruction from a host apparatus, a seek control for positioning a beam of light at a target track designated on a medium. The seek control is divided into a long seek in which the number of tracks up to a target track is more than e.g., 50 tracks and a short seek in which it is less than 50 tracks. In the long seek, a carriage is first sought at a high speed through the speed control of a VCM, and when the number of remaining tracks reaches 50 tracks, the seek is switched to a low speed seek through the speed control of a lens actuator, to move the beam of light to the target track. When the beam of light has reached the target track, control is switched to a position control to lead in the beam of light on the track center, and after the completion of settling, sequence goes to read or write operation. The speed control of the VCM and the lens actuator during the seeking operation is a control following the target speed set in response to the number of tracks up to the target track and has acceleration, constant and deceleration speed profiles. In the short seek in which the number of tracks up to the target track is less than 50 tracks, the low speed seek through the speed control of the lens actuator is initially used and the lead-in operation is carried out when the beam of light reaches the target track. The target speed set in the speed control for the high speed seek and the low speed seek is set in response to the number of remaining tracks to be crossed to reach the target track. For this reason, a track counter is provided which indicates a current position of the beam. The track counter counts the number of track zero-cross pulses generated in response to the detection of zero-crossing of the tracking error signal. The number of tracks up to the target track from the current position is preset in the track counter, which counts down during a seek in the inner or outer direction to at all times indicate the number of tracks up to the target track.
In the conventional optical storage apparatus, however, the tracking error signal obtained during the seek is subjected to a waveform distortion arising from a variation of return light from a physical pit formed on the ID unit. If this waveform distortion occurs at or near the zero-crossing of the tracking error signal, a track zero-cross error detection may take place. The similar track zero-cross error detection may also occur when noise is applied at the timing near the zero-crossing of the tracking error signal. If a track zero-cross error occurs during the seek, the count value of the track counter may undergo an error. In the speed control during the seek, the beam of light speed is detected on the basis of the count value of the track counter, so that if the track counter misses in counting, an erroneous speed is detected, leading to an unstable seek control. In addition, the miscount of the track counter results in an incorrect value of the number of remaining tracks to be crossed to reach the target track, making it impossible to position the beam of light at the target track, bringing about a seek error. The seek error would necessitates a retry seek in which the miss seek track ID is read to recognize the current track to again perform a seek at the target track, resulting in poor access properties.